The present application relates to a storage element suitable for use as a nonvolatile memory in which the direction of magnetization of a storage layer is changed by injecting spin-polarized electrons, and a memory having such storage elements.
High-speed, high-density DRAM is widely used in computers and other information equipments as random access memory.
However, since the information therein disappears when power to DRAM is turned off, there is a demand for nonvolatile memory in which information will not disappear when there is no power.
As one candidate for such nonvolatile memory, a magnetic random access memory (MRAM) in which information is recorded using magnetization in a magnetic material is attracting attention and is currently under development.
In the MRAM, currents are passed through two types of address lines (word lines and bit lines) mutually intersecting approximately at right angles and information is recorded by inverting the magnetization in the magnetic layer of the magnetic storage element at the intersection of address lines by the electric current-induced magnetic field generated from the address lines.
FIG. 1 shows (in perspective view) a schematic diagram of a typical MRAM.
Drain regions 108, source regions 107, and gate electrodes 101 forming selection transistors to select memory cells are formed in portions isolated by element isolation layers 102 in a semiconductor substrate 110 such as a silicon substrate.
Word lines 105 are provided above the gate electrodes 101 and extending in the width direction of the semiconductor substrate 110.
The drain regions 108 are formed so as to be shared by selection transistors on the left and right of the semiconductor substrate 110, and lines 109 are connected to the drain regions 108.
Magnetic storage elements 103 having a magnetic layer the magnetization direction of which is inverted are placed between the word lines 105 and the bit lines 106 that are located above the word lines 105 and extending in the longitudinal direction of the semiconductor substrate 110. These magnetic storage elements 103 are formed of, for example, magnetic tunnel junction (MTJ) elements.
Further, the magnetic storage elements 103 are electrically connected to the source regions via bypass lines 111 located in the longitudinal direction and a contact layer 104 extending below the bypass lines 111.
A current-induced magnetic field is applied to a magnetic storage element 103 by passing currents through a word line 105 and a bit line 106 so as to invert the direction of magnetization in the storage layer of the magnetic storage element 103. As a result, information can be recorded in the magnetic storage element 103.
In a MRAM and other magnetic memories, it is preferable that the magnetic layer (storage layer) in which information is recorded have a constant coercive force to hold recorded information stably.
On the other hand, it is also preferable that a certain amount of electric current pass through address lines to overwrite recorded information.
However, as elements employed in the MRAM are decreased in size, address lines is also formed thinner. As a result, it may be difficult that a sufficiently large current pass through the address lines.
Hence, a memory configured to utilize inversion of magnetization due to spin injection with smaller currents is attracting attention (e.g., Japanese Unexamined Patent Application Publication No. 2003-17782, U.S. Pat. No. 6,256,223, Phys. Rev. B, 54, 9353 (1996), J. Magn. Mat., 159, L1 (1996)).
Inversion of magnetization due to spin injection involves injecting spin-polarized electrons that have once passed through a magnetic material into another magnetic material, thereby causing inversion of magnetization in the other magnetic material.
For example, the direction of magnetization in at least a portion of the magnetic layer of a giant magnetoresistive effect (GMR) element or a magnetic tunnel junction (MTJ) element can be inverted by passing current in the direction perpendicular to the surface of the film of the giant magnetoresistive effect (GMR) element or the magnetic tunnel junction (MTJ) element.
Moreover, spin injection can cause inversion of magnetization without increasing the current, even when the element is very small.
FIG. 2 and FIG. 3 illustrate schematic diagrams of memory configured to utilize the inversion of magnetization due to spin injection as described above, where FIG. 2 is a perspective view, and FIG. 3 is a cross-sectional view.
Drain regions 58, source regions 57, and gate electrodes 51 forming selection transistors to select memory cells are formed in portions isolated by element isolation layers 52 in a semiconductor substrate 60 such as a silicon substrate. Of these, the gate electrodes 51 also serve as word lines extending in the width direction of the semiconductor substrate 60 in FIG. 2.
The drain regions 58 are formed so as to be shared by selection transistors on the left and right of the semiconductor substrate 60 in FIG. 2, and lines 59 are connected to the drain regions 58.
Storage elements 53 having a magnetic layer the magnetization direction of which is inverted are placed between the word lines 55 and the bit lines 56 that are located above the word lines 55 and extending in the left-right direction of the semiconductor substrate 60 in FIG. 2.
These magnetic storage elements 53 are formed of, for example, magnetic tunnel junction (MTJ) elements. In FIG. 3, two magnetic layers 61 and 62 are illustrated, one of the magnetic layers is a fixed-magnetization layer the magnetization direction of which is fixed, and the other magnetic layer is a free-magnetization layer, that is, a storage layer, the magnetization direction of which changes.
The storage element 53 is connected to a bit line 56 and source region 57 via a contact layer 54 in the height direction of the semiconductor substrate 60 to pass current through the storage element 53, and spin injection can cause inversion in the direction of magnetization in the storage layer.
Memory configured to utilize inversion of magnetization due to spin injection is capable of simplifying the device structure compared with ordinary MRAM, as shown in FIG. 1.
Further, in comparison with ordinary MRAM in which inversion of magnetization is caused using an external magnetic field, the write current will not be increased even when the size of the element is reduced.
In the case of MRAM, write lines (word lines and bit lines) and the storage elements are provided separately and information is written (recorded) into the storage elements by passing a current through write lines and utilizing the generated current-induced magnetic field. As a result, a sufficiently large current for writing information can cause to pass through the write lines.
On the other hand, in memory configured to utilize inversion of magnetization due to spin injection, it is preferable to cause spin injection by passing a current through storage elements thereby inverting the direction of magnetization in the storage layer. Since information is written (recorded) into storage elements by directly passing a current through the storage elements in this way, it is preferable that the memory cells include storage elements connected to selection transistors to select memory cells for writing. Here, the current flowing through a storage element is limited to the current which can be passed through a selection transistor (the selection transistor saturation current).
Thus, it may be necessary to write information using a current equal to or less than the saturation current of selection transistors. Consequently, current passed through storage elements may be reduced by improving the efficiency of spin injection.
Further, a large magnetoresistive change rate may be secured for increasing the read signal; to this end, it is effective to use a storage element configuration in which an intermediate layer in contact with both sides of the storage layer is a tunnel insulating layer (tunnel barrier layer).
When using a tunnel insulating layer as an intermediate layer in this way, the amount of current passed through the storage element may be limited to prevent dielectric breakdown of the tunnel insulating layer. Accordingly, the current during spin injection may need controlling.